1. Field of the Invention
The present invention is related generally to formulation and fabrication of doped semiconductor materials and more specifically to isoelectronic co-doping of semiconductor materials.
2. Description of the State of the Art
It is conventional to formulate and fabricate semiconductor materials by doping crystal lattice materials whereby a small amount of an element belonging to one column on the periodic table of the elements, i.e., one number of conduction or outer shell electrons, is replaced with a small amount of material from a different column or group on the periodic table, i.e., a different number of conduction or outer shell electrons, usually one column or group removed, i.e., one more or one fewer outer shell electrons. For example, silicon (a group IV material) is often doped with a small amount of boron (a group III material) to make electronic devices. It is also conventional to use various alloys to form the semiconductor material, i.e., substitutions on lattice sites by elements from the same group in the periodic table, to obtain whatever semiconductor characteristics are needed or desired, such as band gap, crystal lattice constant, mobility, and the like. Solar cells and light emitting semiconductors are usually made with semiconductor materials that have direct bandgaps (i.e., no change in momentum is required for an electron to cross the bandgap between the valence and the conduction energy bands). There have been, and still are, a number of attempts to fabricate solar cells, photodiodes, and light emitting semiconductors using silicon, which is an indirect bandgap material, to open the possibility of combining the benefits of silicon integrated circuits with the energy and communications capabilities of light, but such efforts have seen only limited success.
In the solar cell realm, there are many efforts to obtain a semiconductor material with a bandgap close to 1.0 eV and that is compatible with, i.e., lattice matched to, other semiconductor materials with higher and lower bandgaps to form an integral component of a monolithic quadruple junction solar cell. For example, it is known from mathematical modeling that, for optimum energy absorption from the solar spectrum using four discrete solar cells, the four discrete cells should have bandgaps in the sequence of 1.9 eV, 1.42 eV, 1.05 eV, and 0.67 eV, respectively. Such a monolithic, four-junction, 1.9 eV/1.42 eV/1.05 eV/0.67 eV solar cell structure could attain solar energy conversion efficiency of forty percent (40%) AM1. As mentioned above, solar cells are usually made of group III and group V semiconductor materials, such as gallium (Ga) and arsenic (As), which have direct bandgaps that facilitate absorption and conversion of light energy to electricity and because GaAs wafers for use as lattice matched substrates are readily available. Such group III and group V semiconductor materials with bandgaps at or near the 1.9 eV and 1.42 eV levels with good electric device qualities have been available for some time, e.g., InGaP (1.90 eV) and GaAs (1.42 eV). Germanium (Ge) with a bandgap of 0.67 eV, while it is a group IV material, can never-the-less be used for the substrate part of a monolithic, quadruple junction solar cell, because the substrate can be made thick enough to absorb light energy in spite of the indirect bandgap.
However, making a light absorbent, semiconductor material for the 1.05 eV bandgap cell has been an elusive goal. While 1.05 eV bandgap materials can be made, there have not been any suitable 1.05 eV bandgap materials that have acceptable carrier mobilities and other electronic properties and that can be lattice-matched to the other semiconductor materials for the other cell junctions, as discussed above, such as GaAs and Ge.
Among the semiconductor materials systems that have been investigate for a suitable 1.05 eV cell layer is GaInNAs. The interest in this system was sparked initially y the surprising observation by Weyers et al, Jpn. J. Appl. Physics 31(1992) pp. L853, that the dilute alloy GaAs1-xNx has a giant conduction band bowing, Neugebauer, et al, Phys. Rev. B. 51(1995) pp. 10568, and then by the demonstration by Kondow et al., in 1996 that the quaternary alloy Ga0.92 In0.08N0.03As0.97 could be grown lattice matched to GaAs with a b gap of 1.0 eV J Cryst. Growth 164 (1996) pp. 175. The 8% In was used by Kondow et al., al ng with the Ga, As, and N, to achieve lattice matching with a GaAs substrate. However, the photoluminescense of Ga0.92In0.08N0.03As0.97 is poor. In spite of dilute GaAs1-xNx having the desired 1.0 eV bandgap and the ability to lattice match it to GaAs by the addition of In to the alloy, the very poor electron mobilities in such dilute GaAs1-xNx based alloys has inhibited an further significant progress toward using such an alloy in a photoelectric device, such a monolithic, quadruple junction, solar cell.
Accordingly, a specific object of this invention is to modify a dilute GaAs1-xNx alloy in a manner that not only maintains a bandgap close to 1.05 eV and is lattice matched to GaAs, but also that increases the electronic device quality of the alloy enough to make it not only useful, but also beneficial, as a solar cell semiconductor material.
A more general object of this invention is to expand the repertoire for available choices of semiconductor compounds and alloys that have bandgap/lattice constant paired values suited for expitaxial growth of device structures that implement design principles closest to the ideal.
Another specific object of this invention is to tailor semiconductor materials, such as GaP, to have bandgaps that are close to optimum solar radiation absorption for two junction and three junction tandem solar cells while also being lattice matched to silicon substrates.
Another specific object of this invention is to fabricate and tailor semiconductor materials for active layers of LED""s and laser diodes with bandgaps that produce light in wavelengths that are particularly suitable for fiber optic transmission, such as 1.55 xcexcm or 1.3 xcexcm and which are lattice matched to common semiconductor substrate materials, such as Si or GaAs.
A further specific object of this invention is to fabricate and tailor semiconductor materials for LED""s or laser diodes to have bandgaps that produce certain specific colors of light in the red and near infra-red wavelength regions and that are lattice matched to common semiconductor substrate materials, such as Si, GaAs, and GaP.
Yet another object of this invention is to fabricate and tailor semiconductor materials with bandgaps that absorb infrared radiation to produce electricity and that are lattice matched to desired substrate materials, such as InP.
Additional objects, advantages, and novel features of the invention are set forth in part in the description that follows and will become apparent to those skilled in the art upon examination of the following description and figures or may be learned by practicing the invention. Further, the objects and the advantages of the invention may be realized and attained by means of the instrumentalities and in combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described herein, a method of this invention may comprise modifying a semiconductor compound or alloy comprising host atoms in a crystal lattice to have a lower bandgap than the bandgap of the semiconductor compound or alloy prior to such modification. Such bandgap lowering can be achieved by isoelectronically co-doping the semiconductor compound or alloy with a first isoelectronic dopant comprising atoms that form isoelectronic traps in the host crystal lattice, which behave as deep acceptors, and with a second isoelectronic dopant comprising atoms that form isoelectronic traps in the host crystal lattice, which behave as deep donors. For example, a semiconductor material with a host crystal lattice comprising Group III and V host atoms, such as GaAs with a bandgap of 1.42 eV, can be isoelectronically co-doped with N and Bi to lower the bandgap to anything between 1.42 eV and about 0.8 eV. Other examples include isoelectronically co-doping GaP with N and Bi, InP with N and Bi, GaInP with N and Bi, InGaAs with N and Bi. Other combinations are also feasible.
This ability to modify bandgaps of semiconductor alloys without adversely affecting carrier mobilities and other properties provides the ability to fabricate material with desired bandgaps that can also be lattice matched to substrate and other semiconductor materials where such combinations of lattice matching and desired bandgaps were not available prior to this invention. Accordingly, this invention includes semiconductor devices, such as solar cells and LED""s or laser diodes that have not been available before this invention. For example, a monolithic, quadruple junction solar cell, according to this invention, may include a first or bottom cell comprising Ge with a bandgap of about 0.67 eV, a second cell comprising GaAs that is isoelectronically co-doped GaAs:N:Bi with a bandgap of about 1.05 eV, a third cell comprising GaAs with a bandgap of about 1.42 eV, and a fourth or top cell comprising InGaP with a bandgap of about 1.90 eV. This structure can be fabricated on either a Ge or GaAs substrate, if desired. Other examples include isoelectronically co-doped GaP in one or more cells, such as GaP:N:Bi (bandgap of 1.75 eV) or a combination of GaP:N:Bi (bandgap of 1.55 eV) and GaP:N:Bi (bandgap of 2.04 eV) fabricated on a Si substrate and junction (bandgap of 1.1 eV).
LED""s and laser diodes with active (light-emitting) MQW lawyers of, for example, isoelectronically co-doped GaAs:N:Bi:In and GaAs barrier layers, can be fabricated on GaAs substrates with GaInP cladding layers. Similarly, an active region of isoelectronically co-doped GaP:N:Bi can be fabricated on GaP substrates and with GaP barrier layers. In another example, laser diodes with isoelectronically co-doped GaP:N:Bi:In MQW""s separated by GaP barriers, and with AlxGa1-xxe2x88x92yP cladding can be fabricated on Si substrates with multiple, step-graded, layers of GaP1-xxe2x88x92yNxBiy between the cladding and Si substrate to reduce lattice mismatch strain.